Small gas turbine engines are particularly sensitive to the fuel flow rate into the combustor of the engine. Accordingly, such engines require an especially accurate and predictable fuel control. For electronically based fuel control systems, this requires accuracy between the electronic control and the actual fuel flow. However, elaborate fuel measuring systems with feedback control are not always practical in systems, such as small gas turbine engines, that require low fuel flows and/or have low weight and cost requirements. Thus, for such systems, a typical practice is to use a servo valve 10 in an open loop system, such as shown in FIG. 1, wherein a fuel flow command signal 12 into a controller 14 produces a current input signal 16, typically in the range of 0 to 150 ma, which is converted to a metered fuel flow 18 from the servo valve 10, typically in the range of 20 to 400 pph. The metered fuel flow 18 is directed into a combustor 20 by one or more combustor nozzles 22. The metering of the fuel flow 18 is provided by a current driven torque motor 24 which controls the position of a clevis or flapper 26 on a metering valve 28 in the servo valve 10, thereby producing an orifice area that is proportional to the amplitude of the current input signal 16. This can produce a metered fuel flow 18 proportional to the current input signal 16 if the pressure drop .DELTA.P.sub.s of the fuel flow across the metering valve 28 is controlled to a fixed or predictable value by either a restricting or bypassing pressure drop control valve 30. However, for some systems, this arrangement does not provide the required accuracy and predictability.
Thus, there is a need for a new and improved fuel control system that can provide the predictability and accuracy required by some engine systems, such as small gas turbine engines. There is a further need for such a fuel control system that does not add undue weight and/or complexity and/or cost.